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PHOTOS: Inside Nick Jonas' 21st birthday weekend extravaganza

(Yahoo!) - Leave it to the Jonas Brothers to try and upstage the big fight.

Nick, Joe, and Kevin hit up Sin City this weekend with their respective loves to celebrate Nick's arrival into adulthood — or, at least, the fact that he's now able to legally drink!

The Wynn Las Vegas was their home base for the weekend, and they took advantage of everything the mega resort had to offer.

They festivities kicked off on Saturday night as they visited the cast of aquatic acrobatic show "Le Reve — The Dream."

Afterwards, the brothers were joined by their significant others for an intimate dinner at Andrea's. Nick and his former Miss USA and Miss Universe girlfriend, Olivia Culpo, 21, were seen being playful and cuddly throughout the meal while Kevin doted on his expectant wife Danielle. The restaurant provided signature chopsticks with each of their names engraved on them at the table. The group chowed down on sushi rolls, including crispy tuna, special cucumber roll, and spicy tuna shrimp tempura, in addition to lettuce cups, New York Strip steak, black cod, Jidori chicken, and the restaurant's signature smoked salmon.

On Sunday, the group took it easy during the day, playing golf and watching football before the big birthday night out.

A group of 25 of Nick's closest friends and family gathered at Encore hotel steakhouse Botero at around 10:30 p.m. The entourage was seated on the patio to enjoy the warm Las Vegas evening, indulging in the restaurant's celebrated steaks and splitting several sides including truffle macaroni & cheese, ricotta gnocchi, and wild mushrooms.

At midnight, Nick watched his iPhone's date change over to officially ring in his birthday. The group erupted in applause, while Nick was presented with a four-tier cake by the staff. Joe, 24, and Kevin, 25, sang "Happy Birthday" while Olivia snapped pictures on her phone. The couple then shared a kiss over a celebratory glass of champagne.

The brothers executed a family tradition as Nick made the first cut of his cake and then handed the knife over to Danielle, who has the next upcoming birthday in the group, to make the final cut before dessert was served to everyone. Kevin was overheard explaining the tradition to their friends at the table.

Fifteen minutes after midnight, Nick led his party to the red carpet at XS nightclub, where the brothers all posed with their leading ladies — Nick and Olivia, Kevin and Danielle, and Joe with model girlfriend Blanda Eggenschwiler. Nick shared with media that he's looking forward to some music and acting projects that he has lined up in the year ahead, but also being able to finally just have a beer at the end of the day!

Their entourage of 20 was escorted down the nightclub's signature golden staircase just after 1 a.m. with 30 waitresses flanking them on both sides waving light wands to direct them through the crowd. They arrived on stage and sat next to the night's resident headlining DJ, Avicii, who even grabbed a photo with the famous family before starting his set. A second birthday cake was brought out by the XS model waitresses, themed in true Vegas style with seven tiers of poker chips, card suits, and black-and-red colors throughout.

In the gift department, Olivia gave Nick a necklace, while Joe gave his younger bro a pair of Frank Sinatra's tap shoes.

And like most people celebrating their birthdays in Sin City, Nick and his pals partied into the wee hours of the morning. Other celebs spotted in XS throughout the night (not together) included a newly single Liam Hemsworth, Enrique Iglesias, and Lil Jon.